Switchable electro-optical laminates

ABSTRACT

The present invention is directed to an electrically switchable laminate construction for applications including smart windows, and other uses and applications in which light management is desired. The electro-optical laminate construction has scattering and transparent modes of operation for dynamically controlling electromagnetic radiation flow.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/401,974 filed on Mar. 27, 2003.

FIELD OF THE INVENTION

The present invention is directed to an electrically switchable filmconstruction for applications including smart windows, graphics, officepartitions, green houses and other applications in which lightmanagement is desired. The electro-optical laminate construction has twoor more of transparent, reflective, scattering and opaque modes ofoperation for controlling electromagnetic radiation on demand. Thelaminate construction can be customized to fit various applications.

BACKGROUND OF THE INVENTION

Electrically switchable structures also referred to as “intelligent”glazing structures, or “smart windows”, have been used to controlelectromagnetic radiation in buildings and vehicles. Such structureshave light transmission characteristics that can be electricallycontrolled during the course of the day, or year, in order to meetlighting needs, minimize thermal load on heating and/or cooling systems,and provide privacy within the interior spaces of buildings andvehicles.

There are two general categories of chromogenic switchable glazing orsmart windows, namely: non-electrically activated switchable glazingsand electrically activated switchable glazings. The non-electricallyactivated types of chromogenic switchable glazing are based onphotochromics, thermochromics and thermotropics. The most commonelectrically activated types of chromogenic switchable glazing are basedon polymer dispersed liquid crystals (PDLC), dispersed particle systems(DPS) and electrochromics.

Electro-optical laminate structures having total-reflection,semi-transparent and totally transparent modes of operation for improvedcontrol over the flow of electromagnetic radiation have been developed.Such structures comprise one or more cholesteric liquid crystal (CLC)electromagnetic radiation polarizing panels.

CLC polarizers are used in light valves and electro-optical glazing, orsmart window constructions to control light. Such constructionstypically comprise two rigid sheets of glass on either side of the CLClayer. The CLC layer comprises crosslinkable or polymerizable materialmixed with non-crosslinkable liquid crystals and chiral dopants. Eachsheet of glass is covered with a transparent, electrically conductivecoating to which electrical connections are attached. The structure istypically mounted within a frame.

In the “normal” mode, the CLC layer appears opaque. The liquid crystalsare oriented in multiple directions and scatter light striking the CLClayer, making the device appear opaque. When the device window isswitched on, the electrical field between the two conductive coatingsforces the liquid crystals to reorient themselves parallel to eachother. The CLC layer then appears transparent, and light passes throughthe device without scattering. U.S. Pat. Nos. 5,437,811 and 5,691,795,and International Publications WO 93/23496 and WO 0060407 describeelectro-optical structures that operate in the “normal” mode.

Electro-optical devices incorporating CLC polarizers may also beconfigured to operate in “reverse” mode, wherein the device initiallyappears clear and is switched to opaque. When no electrical field isapplied to the CLC layer, light passes through the device withoutscattering. Upon application of the electrical field, the liquidcrystals reorient themselves to scatter light. U.S. Pat. Nos. 5,437,811and 5,691,795, and International Publication WO 93/23496 describeelectro-optical structures that operate in the “reverse” mode.

Electro-optical laminate structures may also be configured to operate in“reflective” mode, wherein the device is electrically switched betweenlow and high reflectivity. U.S. Pat. Nos. 5,251,048; 5,384,067;5,668,614; 5,940,150 and 6,072,549 and International Publications WO98/38547 and WO 99/63400 describe electro-optical structures thatoperate in “reflective” mode.

In “bistable” mode, the liquid crystals are stable in both the clearstate and the scattering state. The electro-optical structure requireselectrical power only during switching. Power is not required tomaintain either the clear state or the scattering state. U.S. Pat. Nos.5,691,795 and 5,748,277 describe “bistable” electro-optical structures.

There is a need for switchable electro-optical devices improved opticalproperties and increased stability. Additionally, there is a need forelectro-optical laminate structures that can be readily customized tofit various applications.

SUMMARY OF THE INVENTION

In a first aspect, the present invention is directed to a bistableelectro-optical device comprising: a first transparent flexiblesubstrate, having an interior and exterior surface; a second transparentflexible substrate, having an interior and exterior surface; atransparent electrically conductive layer on the interior surface ofeach of the first and second flexible substrates; a cholesteric liquidcrystal material comprising a nematic liquid crystal material havingnegative dielectric anisotropy, a chiral dopant and an ionic additive,wherein the cholesteric liquid crystal material is positioned betweenthe electrically conductive layers of the first and second transparentflexible substrates; wherein the first and second transparent flexiblesubstrates are spaced apart a predetermined distance.

In another aspect of the present invention, the electro-optical deviceincludes a liquid crystal material comprising a polymer network havingcholesteric liquid crystals stabilized and supported therein. In oneembodiment, the polymer network is formed from a polymerizable monomeror polymer having more than one functional group.

The electro-optical laminate structure of the present invention mayinclude a grating surface adjacent to the CLC material for alignment ofthe liquid crystals.

In one embodiment, the CLC layer of the electro-optical laminatestructure comprises individual cells having cell walls. Both the CLCmaterial within the cells and the cell walls respond to an appliedelectrical field.

In one embodiment of the invention, the electro-optical laminatestructure has a protective layer on at least one of the exteriorsurfaces of the transparent substrate.

The electro-optical laminate structure can be mounted within a rigidframe and applied to an existing window structure to control lightimpinging or passing through the window structure. The electro-opticallaminate structures are useful for applications in homes, schools,offices, factories, as well as in automobiles, airplanes and trains toprovide privacy, brightness control, and to reduce thermal loading onheating and cooling systems employed therein.

The electro-optical laminate structure may be provided in a rollconfiguration. In addition, the glazing structure may be supplied in along, continuous sheet that can be cut to any desired dimension. Thisconfiguration enables easy customization of electro-optical laminatestructure for light management application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating electrical switching of theelectro-optical device between the focal conical and planar texture.

FIG. 2 is a cross-sectional view of a switchable laminate of the presentinvention, including two transparent film substrates.

FIG. 3 is a diagram illustrating dual frequency switching of a bistableelectro-optical device between the focal conical and planar texture.

FIG. 4 is a cross-sectional view of the switchable laminate structure ofFIG. 2 with a grating surface on one of the flexible film substrates.

FIG. 5 is a cross-sectional view of the switchable laminate structureillustrating alignment of the CLC molecules.

FIG. 6 is a cross-sectional view of the switchable laminate structurewherein individual cells are formed within the CLC layer.

FIGS. 7A and 7B are top views of the switchable laminate structure withindividual cells formed within the CLC layer.

FIG. 8 is a cross-sectional view of the switchable laminate structure ofthe present invention, including a functional layer overlying theflexible film.

DETAILED DESCRIPTION OF THE INVENTION

The laminate structures of the present invention are electricallyswitched between an opaque state and a transparent state. In the opaquestate, the CLC molecules scatter light because the helically twistedmolecules have randomly oriented axes. This is known as a focal conicaltexture. In the transparent state, the CLC molecules are alignedparallel to the substrate. No reflecting or scattering of light in thevisible spectrum occurs. This is known as a planar texture. In abistable device as shown in FIG. 1, the CLC molecules are switched fromthe focal conical texture to the planar texture by applying an electricfield to the CLC material. The planar structure is maintained(stabilized) when the electric field is removed. By applying an electricfield to the CLC molecules in the planar texture, the CLC molecules areswitched back to the focal conical texture. The focal conical texture ismaintained when the electric field is removed.

Referring to FIG. 2, the illustrative embodiment of the electro-opticallaminate structure of the invention will be described. Theelectro-optical laminate structure 10 generally comprises a CLC material12 interposed between a pair of optically-transparentelectrically-conductive layers 14 a and 14 b supported upon a pair ofspaced-apart transparent, flexible polymeric films 16 a and 16 b,respectively, the perimeter edges of which are sealed, and across whicha voltage is applied under the control of a microcontroller (not shown).Spacers 18 may be including within CLC material 12 to maintain the spacebetween the optically transparent electrically-conductive layers 14 aand 14 b.

The flexible polymeric films 16 a and 16 b are transparent. As usedherein the term “transparent” means that the film does not absorb asignificant amount visible radiation and does not reflect a significantamount of visible radiation, rather, it is transparent to visibleradiation. Examples of polymer films useful as the flexible substratelayer include films made of polyolefin, polyester, polyvinyl chloride,polyvinyl fluoride, polyvinylidene difluoride, polyvinylidene chloride,polyacrylate, polycarbonate, polyurethane, etc., and combinationsthereof. In one embodiment, the flexible films comprise PET films.

In one embodiment, the transparent substrates 16 a and 16 b compriseglass panels. In another embodiment, the transparent substrates compriserigid polymeric films.

The transparent electrically conductive layers 14 a and 14 b maycomprise indium tin oxide (ITO), silver, zinc oxide or other opticallytransparent conductive polymer or like film coating. Chemical vacuumdeposition, chemical vapor deposition, evaporation, sputtering, or othersuitable coating techniques may be used for applying the conductivelayer 14 to the flexible polymeric film 16. In addition, commerciallyavailable inorganic conductive polymeric films, including ITO coatedpolyethylene terephthalate (PET) film from Sheldahl, Inc., may be used.

Conductive organic or polymeric films may also be used. These conductivematerials can be coated onto the flexible film substrates by knownprocesses, including conventional wet coating, spray coating, dipcoating, printing, screen printing and lamination. Commerciallyavailable transparent conductive polymeric films include Orgacon ELpolyethylene dioxithiophene (PEDOT) films from Agfa-Gevaert. Thetransparent conductive film substrate may also have a coating or barrierlayer thereon to reduce oxygen and/or moisture permeation into theglazing structure.

In one embodiment, the conductive layers are treated to provide forsurface alignment of the liquid crystal molecules parallel to the planeof the flexible transparent films, e.g., by providing the conductivelayers with rubbed polyimide layers, sputtered SiO_(x), PEDOT, ortreating them with a surfactant or chemicals. This has the effect ofimproving transmission and response time in some glazing structures inthe field-off condition. In some applications, no alignment layer isrequired.

Electrical leads are attached to the conductive layers 14 a and 14 b. Avoltage source is shown connected to the conductive layers in order toswitch the CLC layer between different optical states by application ofan electric field pulse. The voltage source may be an AC voltage sourceor a DC-AC inverter and a battery. In addition, the switching power maybe supplied by a photovoltaic device that converts solar power toelectrical power.

In one embodiment, the CLC material 12 comprises nematic liquidcrystals, a chiral dopant and an ionic additive. The CLC material mayalso include a polymer matrix formed from a polymerizable monomer orpolymer. The polymer matrix stabilizes or supports the nematic liquidcrystals.

Suitable nematic liquid crystals and chiral additives are commerciallyavailable and would be known to those skilled in the art in view of thisdisclosure.

Suitable chiral nematic (i.e., cholesteric) liquid crystals materialsare disclosed in, for example, U.S. Pat. No. 6,049,366, InternationalPublications WO 00/60407, WO 99/6340 and WO 98/38547, the entiredisclosures of which are incorporated herein by reference. Specificnematic liquid crystalline materials include: p-azoxyanisole,p-azoxyphenetole, p-butoxybenzoic acid, p-methoxy-cinnamic acid,butyl-p-anisylidene-p-aminocinnamate, anisylidene p-amino-phenylacetate,p-ethoxy-benzal-amino-α-methyl-cinnamic acid,1,4-bis(p-ethoxybenzylidene)cyclohexanone, 4,4′-dihexyloxybenzene,4,4′-diheptyloxybenzene), anisal-p-amino-azo-benzene, anisaldazine,α-benzene-azo-(anisal-α′-naphthylamine), n,n′-nonoxybenzetoluidine;anilines of the generic group (p-n-alkoxybenzylidene-p-n-alkylanilines),such as p-methoxybenzylidene p′-n-butylaniline,p-n-butoxybenzylidene-p′-aminophenylacetate,p-n-octoxybenzylidene-p′-aminophenylacetate,p-n-benzylideneproprionate-p′-aminophenylmethoxide,p-n-anixylidene-p′-aminophenylbuterate,p-n-butoxybenzylididene-p′-aminophenylpeatoate and mixtures thereof.Conjugated cyano-organic compounds include7,7′,8,8′-tetracyanoquinodimethane (TCNQ),(2,4,7,-trinitro-9-fluorenylidene)-malono-nitrile (TFM),p-[N-(p′-methoxybenzylidene)amino]-n-butyl-benzene (MBBA),p-[N-(p′-ethoxybenzylidene)amino]-butylbenzene (EBBA),p-[N-(p′-methoxybenzylidene)amino]phenyl butyraten-butyl-p-(p′-ethoxyphenoxycarbonyl)phenylcarbonate,p-methoxy-p′-n-butylazoxybenzene, p-ethoxy-p′-n′-butylazobenzene,p-[N-(p′-methoxybenzylidene)amino]benzonitrile (BBCA),p-[N-(p′-methoxybenzylidene)amino]benzonitrile (BBCA),p-[N-(p′-hexylbenzylidene)amino]benzonitrile (HBCA), pentylphenylmethoxybenzoate, pentylphenylpentyloxy benzoate, cyanophenylpentyl benzoate,cyanophenylheptyloxy benzoate, cyanophenyloctyloxy benzoate,cyanophenylmethoxy benzoate, and the like.

Nematic liquid crystals frequently comprise cyanobiphenyls, and may bemixed with cyanoterphenyls and with various esters. There arecommercially available nematic type liquid crystal mixtures, such asliquid crystal mixture “E7” (Licrilite™ BL001 from E. Merck, Darmstadt,Germany, or its subsidiaries such as EM Industries, Hawthorne, N.Y. andMerck Industrial Chemical, Poole, England) that is a mixture of (byweight), 51% 4′-n-pentyl-n-cyanobiphenyl (5CB), 21%4′-n-heptyl-n-cyanobiphenyl (7CB), 16% 4′-n-octoxy-4-cyanobiphenyl, 12%and 4′-n-pentyl-4′-n-pentyl-4-cyanoterphenyl that has a crystal tonematic liquid crystal phase transition temperature of −10° C. and aliquid crystal to isotropic phase transition temperature of 60.5° C.

Illustrative of other such commercial liquid crystal mixtures are thefollowing: E-31 is a proprietary mixture of cyanobiphenyls and anon-cyano biphenyl ester available from E. Merck, supra, and having acrystal to nematic crystal phase transition temperature of −9° C. and aliquid crystal to isotropic phase transition temperature of 61.5° C.E-44 is a proprietary mixture of cyanobiphenyls, a cyanoterphenyl and anon-cyano biphenyl ester available from E. Merck, supra, and having acrystal to nematic liquid crystal phase transition temperature of −60°C. and a liquid crystal to isotropic phase transition temperature of100° C. E63, from E. Merck, supra, is a liquid crystal mixture that issimilar to the E7 with added cyclohexanes. It contains: significantamounts of the commonly known liquid crystal component 5CB, 7CB, lesseramounts of 5CT, lesser amounts of Benzonitrile-4-(4propyl-1-cyclohexen-1-yl), commonly known as PCH3, lesser amounts of4-carbonitrile-4′(4-pentyl-1-cyclohexen-1-yl)-1,1′-biphenyl, commonlyknown as BCH5, and still lesser amounts of [1,1′-Biphenyl]-4-carboxylicacid, 4′-heptyl-4′-cyano[1,1′-biphenyl]-4-yl ester, commonly known asDB71. K-12 is 4-cyano-4′-butylbiphenyl and has a crystal to nematicliquid crystal phase transition temperature of 48° C. K-18 is4-cyano-4′-hexylbiphenyl and has a crystal to nematic liquid crystalphase transition temperature of 14.5° C. and a liquid crystal toisotropic phase transition temperature of 29° C. K-21 is4-cyano-4′-heptylbiphenyl and has a crystal to nematic liquid crystalphase transition temperature of 30° C. K-24 is 4-cyano-4′-octylbiphenyland has a crystal to smectic A liquid crystal phase transitiontemperature of 21.5° C., a smectic C to nematic liquid crystal phasetransition temperature of 33.5° C. and a nematic liquid crystal toisotropic phase transition temperature of 40.5° C. M-15 is4-cyano-4′-pentoxybiphenyl and has a crystal to nematic liquid crystalphase transition temperature of 48° C. and a liquid crystal to isotropicphase transition temperature of 68° C. M-18 is 4-cyano-4′-hexoxybiphenyland has a crystal to nematic liquid crystal phase transition temperatureof 57° C. and a liquid crystal to isotropic phase transition temperatureof 75.5° C. M-24 is 4-cyano-4′-octoxybiphenyl and has a crystal tosmectic A liquid crystal phase transition temperature of 54.5° C., asmectic A to nematic liquid crystal phase transition temperature of67.0° C. and a nematic to isotropic phase transition temperature of80.0° C. Other Licrilite™ liquid crystal mixtures include BL003, BL004,BL009, BL011, BL012, BL032, BL036, BL037, BL045, BL046, ML-1001,ML-1002, as well as TL202, TL203, TL204 and TL205, all obtainable fromE. Merck, supra.

TOTN404, available from Hoffman-LaRoche, Basel, Switzerland and Nutley,N.J., is a liquid crystal mixture similar to E7 but with addedpyrimidines. It contains approximately 30 weight percent of4-carbonitrile,4′-pentyloxy-1,1′-biphenyl commonly known as 5OCB, 14weight percent of 4-carbonitrile,4′-octyloxy-1,1′-Biphenyl, commonlyknown as 8OCB, 10 weight percent of4-carbonitrile-4″-pentyl-1,1′,4′,1″-terphenyl, commonly known as 5CT, 10weight percent of 4-(4-pentyl-2-pyrimidimyl)-benzonitrile, commonlyknown as RO-CP-7035, 20 weight percent of4-(4-heptyl-2-pyrimidimyl)benzonitrile, commonly known as RO-CP-7037,and 15 weight percent of4-[5-(4-butylphenyl)-2-pyrimidinyl]benzonitrile, commonly known asRO-CM-7334.

ROTN-570, available from Hoffman-LaRoche is a cyanobiphenyl liquidcrystal mixture comprises 51 weight percent of4-cyano-4′-pentylbiphenyl, 25 weight percent of4-cyano-4′-heptylbiphenyl, 16 weight percent of4-cyano4′-octyloxybiphenyl, and 8 weight percent of4-cyano-4′-pentyl-p-terphenyl. Other desirable liquid crystal mixturesinclude TNO623 and TN10427, both from Hoffman-LaRoche.

Commercially available nematic liquid crystal materials from SlichemLiquid Crystal Company of China include 6F10100, and TEB50.

Useful chiral additives include cholesteryl halides, cholesteryl alkylesters including cholesteryl acetate, cyanobiphenyl derivatives such as4-cyano4′-(2-methyl) butylbiphenyl and C15 and CB15 from Merck. Usefulchiral compounds also include ZLI-4571 and ZLI-4572 from Merck.

In one embodiment, the CLC material 12 comprises a polymer matrix havingnematic liquid crystals stabilized or supported therein. The polymermatrix is generally formed by polymerization or crosslinking of at leastone polymerizable monomer or crosslinkable polymer with non-reactivenematic liquid crystals, and a chiral additive. Polymerization of theliquid crystal mixture is initialized in any suitable manner, as by UVradiation, thermally, etc., depending upon the polymer used. The liquidcrystal mixture may also contain a surfactant and/or dye.

In one embodiment, the cholesteric liquid crystal material comprisesabout 90-99% by weight nematic liquid crystal material, about 0.5-3% byweight of chiral dopant and about 0.05-0.5% by weight of ionic additive.

In one embodiment, the polymer matrix is formed from crosslinking amixture of a crosslinkable polymer or monomer(s), a non-crosslinkableliquid crystal(s) and chiral dopant(s). A liquid crystal polymerstabilized cholesteric texture (PSCT) is formed when a small amount of aUV crosslinkable polymer in its liquid crystal phase and aphotoinitiator are mixed with a cholesteric liquid crystal (CLC) whosepitch is tuned to the infrared region. The crosslinkable polymerconcentration is typically in the range of about 0.1 to about 5.0% byweight of the total CLC mixture. U.S. Pat. Nos. 5,384,067; 5,437,811 and5,691,795, and International Publication WO 00/60407, the entiredisclosures of which are incorporated herein by reference, disclose suchCLC mixtures. The monomer may be a UV polymerizable monomer such asethylene glycol dimethacrylate. A surfactant may be included in themixture to facilitate uniform coating and desired orientations of theCLC material within the electro-optical structure. The mixture is thencured by exposure to UV light while a voltage or a magnetic field isapplied to align the liquid crystal as well as the polymer molecules inthe direction across the device thickness. With the field applied duringcuring, the CLC molecules are aligned in a planar texture (transparent)after the curing. Subsequent application of an electric field switchesthe CLC molecules into a stable focal conical texture (opaque) that ismaintained when the electric field is removed. The CLC material isformulated to have an intrinsic reflective wavelength in the infraredrange, e.g., 0.7 to 2.0 microns.

Suitable crosslinkable polymer materials include UV curable,thermoplastic and thermosetting polymers. Examples of crosslinkablepolymers include acrylate and methacrylates, vinyl ethers,hydroxyfunctionalized polymethacrylates, urethanes and epoxy systems.Particularly useful polymerizable materials include acrylate andmethacrylate monomers.

In one embodiment, the cholesteric liquid crystal material comprisesabout 90-98% by weight of a nematic liquid crystal material, about 1-3%by weight of a chiral material and about 1-6% by weight of apolymerizable acrylate or methacrylate based resin material having twoor more functional groups.

Useful photoinitiators include benzoin methyl ether, and the Irgacurefamily of photoinitiators including Irgacure 184, 369, 651, 819 and 907,and Darocure 1173 and 4205, all from Ciba Geigy, as well as otherphotoinitiators known to those in art.

In one embodiment, the CLC material is formed from a mixture comprisinga polymerizable liquid crystal material, a non-polymerizable liquidcrystal material and a chiral dopant. Polymerizable liquid crystalsinclude polysiloxane liquid crystal material and acrylate liquid crystalcompounds. Polysiloxane liquid crystal materials are commerciallyavailable from Wacker (Germany) and acrylate based liquid crystalmaterials are commercially available from BASF or EMI (Germany).Suitable non-polymerizable liquid crystal materials include singlecompound liquid crystals such as the K- and M-series from EMI (Germany)and multiple compound liquid crystals such as the E- and ZLI-series fromEMI.

Dual Frequency Switching

In one embodiment, the CLC material comprises a dual frequencyswitchable CLC material. The CLC material comprises a mixture of acholesteric liquid crystal material with a negative dielectricanisotropy, a polymerizable material, and charge transfer agents (bothdonor and acceptor) or a charge transfer complex, or ionic compounds orpolar compounds. If the polymerizable material is a UV curable polymeror monomers, the CLC mixture also includes a photoinitiator. The CLCmaterial has a threshold frequency, which is the point at which theliquid crystal material changes from one optical state to another. Whenan electric field having a frequency greater than the thresholdfrequency is applied to the CLC material, the liquid crystals alignthemselves parallel to the substrate in a planar texture. When anelectric field having a frequency lower than the threshold frequency isapplied to the CLC material, the electro-hydrodynamic instability of theliquid crystals causes the liquid crystal molecules to reorient to focalconical state causing the light to be scattered and the structureappears opaque. FIG. 3 illustrates the dual frequency switching of thebistable glazing structure of this embodiment. The polymerizablecomponent of the CLC mixture helps control the bistability of the CLCmaterial. Curing the polymerizable component in the presence of a highfrequency electric field applied to the CLC material favors thetransparent, planar texture. In order to favor the opaque, focal conicalstate the polymerizable component is cured in the presence of a lowfrequency electric field applied to the CLC material. The opaque stateis also favored by curing the polymerizable component in the presence ofa magnetic field or in the absence of any field.

Examples of useful charge transfer agents include, but are not limitedto the electron donors bis(ethylenedithio) tetrathiafulvalene,bis(methylenedithio) tetrathiafulvalene, bis(trimethylenedithio)tetrathiafulvalene, 4,4′-dimethyltetrathiafulvalene,tetrakis(octadecylthio) tetrathiafulvalene,tetrakis(n-pentylthio)tetrathiafulvalene,tetrakis(alkylthio)tetrathiafulvalene, tetrathiafulvalene, ferrocene,butylferrocene and tris(tetrathiafulvalene) bis(tetrafluoroborate); theelectron acceptors bis(tetra-n-butylammonium)tetracyanophenoquinometanide,2,5-dimethyl-7,7,8,8-tetracyanoquinodimethane,11,11,12,12-tetracyanonapth-2,6-quinodimethane,7,7,8,8-tetracyanoquinodimethane, and tetracyanoquinodimethane; andcharge transfer complexes obtained by reacting an electron donor with anelectron acceptor. Such charge transfer complexes are described in U.S.Pat. No. 6,384,887, the entire disclosure of which is herebyincorporated by reference herein.

Useful ionic compounds include, but are not limited to,1-heptyl-4(4-pyridyl) pyridinium bromide, 1-phenacyl pryridiniumbromide, 2-propylisoquinolinium bromide, 2-propylisoquinoliniumtetraphenyl borate, cetylpyridinium bromide, dodecyl pyridiniumtetraphenyl borate, tetrabutyl ammonium bromide, tetrabutyl ammoniump-toluene sulfonate, tetrabutylammonium hexafluoro phosphate,tetrabutylammonium tetraphenyl borate, tetrahexadecylammonium bromide,tetrahexadecylammonium hexafluorophosphate, tetrakisdecylammoniumbromide, tetrakisdecylammonium hexafluorophosphate,tetrahexadecylammonium tetraphenyl borate, tetrakisdecylammoniymtetraphenyl borate and mixtures thereof.

Useful polar compounds are those molecules with permanent dipoles andinclude, but are not limited to, singly- or multiply-substitutedstraight chain aliphatics, branched aliphatics, cyclic aliphatics,mononuclear or polynuclear aromatics, heteroaromatics, where thearomatic ring incorporates N, S, or O, and polynuclear heteroaromatics,metallocenes, and combinations thereof. The substitutions can be, butare not limited to, any one of the following functional groups:carboxylic acids, aldehydes, ketones, nitriles, isonitriles, halogens,esters, alcohols, thiols, alkyl, phenyl, biphenyl, and combinationsthereof. Examples of specific polar compounds include acetone,chloroform, dichloromethane, 4-cyanobiphenyl, decanoic acid,1-bromohexadecane, hexanophone, 4-hexylbenzoic acid,2-pyridylacetonitrile, ferrocenecarboxylic acid, ferroceneacetonitrile.

In one embodiment, the electro-optical laminate structure comprises abistable device in which the CLC material comprises a nematic liquidcrystal, a chiral compound and an ionic additive. In another embodiment,the bistable device includes a CLC material comprising a nematic liquidcrystal, a chiral compound, an ionic additive and a polymerizablemonomer or polymer.

Dichroic Dyes

In one embodiment, the CLC material contains one or more dichroic dyesto make the glazing structure darker in its opaque state. In thehomeotropic bright state, the dichroic dye molecules substantiallyfollow the orientation of the surrounding liquid crystals, i.e., arealigned perpendicular to the substrate and thus perpendicular to thelight polarization. Because of this alignment, the addition of thedichroic dye to the CLC does not substantially affect the transparencyof the glazing. However, in the focal conical opaque state, theorientation of the dichroic dye molecules follow the helical structureof the cholesteric liquid crystals. Many of the dichroic dye moleculesare aligned parallel to the substrate and thus parallel to the lightpolarization. In this state, the dichroic dye absorbs lightsignificantly, and the opaque state of the glazing is darker.

The conductive layers 14 a and 14 b are physically spaced apart. In oneembodiment, the spacing between the conductive layers is greater thanabout 10 microns. In another embodiment, the spacing is greater thanabout 15 microns, and in yet another embodiment, the spacing is about 20microns. In a further embodiment, the spacing is about 25 microns. It isunderstood that such dimensions may vary from embodiment to embodimentof the invention.

In one embodiment, the CLC layer contains spacers 18. The spacers maycomprise, for example, glass beads, sticky glass beads, polymericmicrospheres and/or microfibers. In one embodiment, the spacers 18comprise glass beads having an average diameter in the range of 5 to 50microns. In another embodiment, the spacers comprise glass beads havingan average diameter in the range of 10 to 30 microns.

In one embodiment, spacers 18 are printed or sprayed onto at least oneof the conductive layers. In another embodiment, the spacers aremicro-patterned onto at least one of the conductive layers by amicro-replication process or by a photo-lithographic process.

In another embodiment of the present invention, the CLC layer comprisesencapsulated CLC material. The encapsulated CLC material can function asspacers, so that separate glass or polymeric spacers are not required.

In one embodiment, the CLC material comprises CLC pigments in anelectrically active carrier fluid, such as low molecular weight nematicliquid crystal fluid. Such CLC materials are described in, for example,International Publication WO 98/38547, the disclosure of which isincorporated herein by reference.

Other embodiments of CLC materials are described in U.S. Pat. Nos.5,251,048; 5,384,067; 5,437,811; 5,668,614; 5,695,682 and 5,748,277, theentire disclosures of which are incorporated herein by reference.

Grating Surface

In one embodiment, the glazing structure of the present inventionincludes a grating surface, also known as a ZBD device, on the innersurface of one or both of the transparent substrates. The gratingsurface comprises a plurality of small surface features, generally lessthan 15 microns in size. Such small surface features include grooves,protrusions, blind holes, and other surface profiles. The grating areasmay be uniform or non-uniform in size, shape, and alignment directions,depending on the desired distortion effect. The grating surface is usedfor alignment and surface tilt. Examples of grating structures includethose described in International Patent Publication WO 01/40853, theentire disclosure of which is hereby incorporated by reference herein.Referring to FIG. 4, the glazing structure 40 comprises CLC material 12interposed between a pair of optically transparent electricallyconductive layers 14 a and 14 b supported upon a pair of spaced-apart,transparent substrates 16 a and 16 b, respectively. Grating surface 20can be applied to the inner surface of conductive layers 14 a, andoptionally 14 b. Alternatively, conductive layer 14 a may be applied tothe inner surface of grating surface 20. In one embodiment, the gratingsurface is the conductive layer itself. For example, the grating surfacecan be formed on a layer of PEDOT by photolithography or by embossingthe conductive layer. Other manufacturing techniques include scoring,printing, lithography, laser ablation and interferographic techniques.Spacers (not shown) are used to uniformly space apart substrates 16 aand 16 b. The spacers can be an integral part of the grating surfacestructure. For example, spacers can be formed by embossingsimultaneously with the formation of the grating surface.

With the grating surface, the surface alignment of the CLC molecules canbe switched between a high tilt orientation and a low tilt orientationrelative to the substrates. By changing the electric pulse polarity, thesurface orientation of the CLC molecules is reversed. The CLC moleculesin the vicinity of the grating surface will respond to the electricpulse polarity to align themselves with perpendicular (high tiltorientation) or parallel (low tilt orientation) to the substrate.Accordingly, the bulk CLC molecules adopt a texture of either the focalconic state where the CLC molecules on the grating surface are alignedhigh tilted, or the planar state where the CLC molecules on the gratingsurface are aligned low tilted. As shown in FIG. 5, the focal conicstate scatters the light, making the panel appear opaque (5A). Theplanar state allows the incident light to pass through without loss whenthe helical pitch of the CLC molecules are selected to reflect lightoutside of the visible band (5B). By switching the polarity of theapplied electric field, a change between the transparent state and theopaque state occurs.

Electro-Optical Structure With Individual Cells

In one embodiment, the CLC material is partitioned into distinctregions, or individual cells in accordance with a predetermined pattern.As shown in FIG. 6, the glazing structure 60 comprised CLC layer 12interposed between a pair of optically-transparentelectrically-conductive layers 14 a and 14 b supported upon a pair ofspaced-apart transparent, flexible polymeric films 16 a and 16 b,respectively, the perimeter edges of which are sealed and across which avoltage is applied. Individual cells 4 are supported by walls 6, whichextend the entire distance between films 16 a and 16 b (includingconductive layers 14 a and 14 b). In one embodiment, the distancebetween the cell walls can be within the range of about 0.1 millimetersto about 10 millimeters. The width of the walls can be within the rangeof about 10 μm to about 1000 μm. Walls 6 facilitate cutting of theglazing structure 60 into any desired shape and dimension whileminimizing liquid crystal leakage and preventing electrical shorting anddelamination.

In one embodiment, the CLC layer is formed from a mixture comprisingliquid crystal material, at least one polymerizable monomer orcrosslinkable polymer, and a photoinitiator. A two-stage polymerizationprocess may be used to form the CLC layer. A photomask of apredetermined pattern is placed over the CLC mixture and exposed toradiation such as UV radiation. Examples of suitable patterns includethose illustrated in FIGS. 7A and 7B.

Only in the irradiated regions does photopolymerization take place, andthe subsequent decrease in the monomer content in the irradiated regionscauses a gradient in monomer concentration. As a result, morephotopolymerizable monomer diffuses into the irradiated regionsdisplacing the CLC. The cell walls are formed in the irradiated regions.By adjusting the polymerizable monomer's diffusion coefficient, UVexposure intensity and time, the illumination dimensions, the polymerconcentration in both the walls and cell areas can be controlled. Asecond exposure, a blanket exposure, i.e., without a mask, produces apolymer stabilized liquid crystal within the individual cells. Due tothe low concentration of polymerizable monomer within the cells, theresulting polymer forms a network within which the CLC molecules arehomogeneous and mobile. Within the cell walls, the polymer is moreconcentrated so that the polymer and CLC molecules are phase separatedinto domains. The CLC molecules are confined in droplets surrounded bythe polymer. The walls of the cells, as well as the CLC material withinthe cells, respond to the externally applied electric field.

Examples of suitable photopolymerizable monomers include acrylic acidand methacrylic acid, esters thereof, each of which contains an alkylgroup, an aryl group, or a cycloalkyl group including three or morecarbon atoms, and halides thereof. Such photocurable monomers are, forexample, isobutyl acrylate, stearyl acrylate, lauryl acrylate, isoamylacrylate, 2-ethylhexyl acrylate, n-butyl methacrylate, n-laurylmethacrylate, tridecyl methacrylate; n-stearyl methacrylate,n-cyclohexyl methacrylate, benzyl methacrylate, isobornyl methacrylate,2-phenoxyethyl methacrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate,2,2,3,4,4,4-hexachlorobutyl methacrylate, 2,2,3,3-tetrachloropropylmethacrylate, 2,2,3,3-tetrachloropropyl methacrylate,perfluorooctylethyl methacrylate, perfluorooctylethyl acrylate, andperchlorooctylethyl methacrylate. Polyfunctional compounds may also beused. Polyfunctional compounds are, for example, ethylene glycoldimethacrylate, bisphenol-A diacrylate, bisphenol-A dimethacrylate,1,4-butanediol dimethacrylate, 1,6-hexanediol dimethacrylate,trimethylolpropane triacrylate, and tetramethylolmethane tetraacrylate.Such monomers and polyfunctional compounds may be used independently orin a combination of two or more.

In one embodiment, the glazing structure includes a barrier layer on theinterior or exterior of the transparent flexible film to reduce waterand oxygen transmission through the flexible film. Conventional barrierlayers including polyvinylidene chloride, polyvinyl alcohol, SiO_(x)and/or ITO may be used.

Referring now to FIG. 8, an embodiment of the electro-optical glazingstructure 80 of the present invention incorporating a functional layer 8is illustrated. Depending on the application for which theelectro-optical glazing structure is used, one or both of the exteriorsurfaces of the glazing structure may require a protective coating orlayer. For example, the glazing structure may be used in an atmospherewhere dew or fog is formed on the glazing structure. The exterior of theglazing structure may accumulate dust or fingerprints, or may besubjected to abrasion. Thus functional layer 8 may comprise, forexample, an anti-fog or moisture barrier layer, an anti-bacterialcoating, an anti-static coating, an abrasion-resistant coating, and/or acoating with self-cleaning properties. A gas barrier layer, UVblocking/filtering layer, anti-reflection layer, infrared reflectionlayer or liquid crystal alignment layer may be included as a functionallayer within the interior or on the exterior of the glazing structure.

The process for making the electro-optical glazing structure of thepresent invention includes the steps of (a) providing two transparentsubstrates coated with a transparent conductive layer, wherein thesubstrates are separated by spacers to create an area between thesubstrates (b) depositing a cholesteric liquid crystal (CLC) mixturecontaining a polymerizable monomer in the area between the substrates,(c) sealing the perimeter of the transparent substrates to contain theCLC mixture with the monomer within the area between the substrates and(d) polymerizing the monomer.

In one embodiment of the present invention, the process is carried outin a substantially continuous operation. The continuous process formaking the electro-optical glazing structure includes the steps of (a)providing two substantially continuous flexible transparent substratescoated with a transparent conductive layer, wherein the flexiblesubstrates are separated by spacers to create an area between theflexible substrates (b) continuously depositing a cholesteric liquidcrystal (CLC) mixture containing a polymerizable monomer in the areabetween the substrates, (c) sealing the perimeter of the flexibletransparent substrates to contain the CLC mixture with the monomerwithin the area between the substrates and (d) polymerizing the monomer.

As used herein, the term “substantially continuous” means, with respectto a component of the process, for example the transparent flexiblesubstrate of the glazing structure, such component is provided in along, continuous condition, such as on a supply roll, from which aplurality of parts may be obtained. The term “substantially” is includedin recognition of the fact that a given supply roll must have a finitelength. With respect to a process, the term “substantially continuous”is used in its conventional meaning, and means that the operation(s)is/are carried without significant interruption or cessation betweensteps.

In one embodiment, the flexible transparent substrates are coated withITO and baked to drive off moisture. The ITO coated surface of one ofthe substrates is then sprayed with glass bead spacers. The liquidcrystal-monomer mixture is then deposited on one of the substrates. Thesecond flexible transparent substrate is laminated to the firstsubstrate so that the liquid crystal-monomer mixture contacts theconductive layer on each of the transparent substrates.

In another embodiment, spacers are included in the CLC mixture and areapplied to the transparent flexible substrate when the CLC material iscoat deposited or coated onto the flexible substrate.

The CLC material can be coated onto the conductive film by any knownmethod suitable for coating liquid materials. For example, the CLCmaterial may be applied to the conductive film by gravure coating,curtain coating, die-coating, printing and screen printing.

The laminate is prepared by polymerizing he liquid crystal-monomermixture either in zero electric field or in an electric field effectiveto align the liquid crystal directors. The polymer network that iscreated in the material may serve to stabilize the light scatteringstate resulting from application of a low electric field pulse and thelight transmitting state resulting from application of a high electricfield pulse.

In one embodiment of the present invention, the electro-optical glazinglaminate is manufactured in a continuous process that produces a long,continuous roll of the laminate structure. This configuration enableseasy customization of electro-optical glazing laminates for lightmanagement application.

Segments of the roll, in the desired dimensions, may be die cut from theroll to produce individual “smart window” glazing structures. Theperimeter of the die cut segment is then sealed to prevent loss of theCLC material and ingress of oxygen and/or moisture into the structure.Sealing may be carried out by, for example, applying a glue or sealantto the perimeter of the segment, by a thermal process or by activating achemically reactive material within the laminate structure.

In one embodiment, the CLC material includes encapsulated epoxy resindispersed therein. The encapsulated epoxy resin may function as thespacers between the conductive layers. Alternatively, the encapsulatedepoxy resin is included in the CLC material along with more conventionalspacers. Upon die cutting the large laminate material or roll intoindividual segments, the epoxy resin proximate to the cut cures, thussealing the perimeter of the laminate segment.

The electro-optical glazing structures described herein can be stackedand laminated together, in virtually any number or ordering, so as toform composite electro-optical glazing structures having more than twooptical states. Such electro-optical glazing structures can be used toconstruct sophisticated window systems capable of providing complexlevels of solar and/or visible radiation control.

EXAMPLES Normal Mode Comparative Examples 1A and 1B

A normal mode cell, light scattering in the field-off condition andoptically clear in the field-on condition, was prepared by using each ofthe following compositions (% by weight listed): Optical PropertiesScattering State - Haze (%) Clear State - Clarity (%) Low Intensity HighIntensity Low Intensity High Intensity Example Formulation (% by weight)Curing Curing Curing Curing 1A 92.9% TEB 50¹ 78.9 77.0 86.0 87.6  5.0%RM82²  2.0% ZLI-4571³  0.1% Irgacure 819⁴ 1B 95.9% TEB 50 18.7 65.9 99.499.4  2.0% RM82  2.0% ZLI-4571  0.1% Irgacure 819¹TEB50: nematic liquid crystal, Slichem Liquid Crystal Company, Ltd,China²RM82: mesogenic monomer, Merck KGaA, Germany³ZLI-4571: chiral compound, Merck KGaA, Germany⁴Irgacure 819, Bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide,photoinitiator, Ciba Specialty Chemicals

The polymerizable composition was vacuum filled into a cell having twoglass substrates coated with indium-tin oxide on their inner surfaces.The glass substrates were separated by 20 micron spacers. The filledcell was irradiated with UV light under different conditions: lowintensity curing, exposing the cell to UV light with intensity of 10mW/cm² at 365 nm for 1000 seconds and high intensity curing, exposingthe cell to UV light with intensity of 100 mW/cm² 365 nm for 500seconds. While the cell was being irradiated, an electric field of 111V60 Hz was applied to the cell.

The optical properties of the cells, % haze in the scattering state and% clarity in the clear state were measured using a BYK-Gardner Haze-GardPlus Instrument in accordance with ASTM D 1003. The electric fieldapplied to the cells at the time of measurement of optical clarity was5.55×10⁶ V/m.

Examples 2A-2F

The following series of normal mode cells were prepared substantially inaccordance with Examples 1A and 1B above, with the exception that thefollowing compositions including monofunctional monomers were used:Optical Properties Scattering State - Haze (%) Clear State - Clarity (%)Low Intensity High Intensity Low Intensity High Intensity ExampleFormulation (% by weight) Curing Curing Curing Curing 2A 92.9% TEB 5023.9 23.9 95.0 95.4  5.0% IBOA⁵  2.0% ZLI-4571  0.1% Irgacure 819 2B95.9% TEB 50 49.1 50.8 97.6 98.2  2.0% IBOA  2.0% ZLI-4571  0.1%Irgacure 819 2C 92.9% TEB 50 93.3 93.8 99.1 99.1  5.0% IBOMA⁶  2.0%ZLI-4571  0.1% Irgacure 819 2D 95.9% TEB 50 95.1 96.1 99.3 99.2  2.0%IBOMA  2.0% ZLI-4571  0.1% Irgacure 819 2E 95.9% TEB 50 — 75.2 — 98.9 2.0% IDA⁷  2.0% ZLI-4571  0.1% Irgacure 819 2F 95.9% TEB 50 — 78.0 —98.3  2.0% IDMA⁸  2.0% ZLI-4571  0.1% Irgacure 819⁵isobornyl acrylate, Ciba Specialty Chemicals⁶isobornyl methacrylate, Ciba Specialty Chemicals⁷isodecyl acrylate⁸isodecyl methacrylate

Examples 3A-3D

The following series of normal mode cells were prepared substantially inaccordance with Examples 1A and 1B above, with the exception that thefollowing compositions including bifunctional monomers were used:Optical Properties Scattering State - Haze (%) Clear State - Clarity (%)Low Intensity High Intensity Low Intensity High Intensity ExampleFormulation (% by weight) Curing Curing Curing Curing 3A 92.9% TEB 50102.0 103.0 99.6 99.1  5.0% EGDMA⁹  2.0% ZLI-4571  0.1% Irgacure 819 3B92.9% TEB 50 101.0 100.0 99.3 99.2  2.5% EGDMA  2.5% IBOMA  2.0%ZLI-4571  0.1% Irgacure 819 3C 92.9% TEB 50 — 98.1 — 98.8  5.0% EGDA¹⁰ 2.0% ZLI-4571  0.1% Irgacure 819 3D 95.9% TEB 50 — 103.0 — 99.0  2.0%EGDA  2.0% ZLI-4571  0.1% Irgacure 819⁹ethyleme glycol dimethacrylate, Aldrich¹⁰ethylene glycol diacrylate

Example 4

A normal mode cell was prepared substantially in accordance with Example3A above, with the exception that flexible transparent substrates wereused. The polymerizable formulation was sandwiched between two 7 mil PETfilms having a conductive indium-tin oxide coating on their innersurfaces. A uniform cell gap of 20 microns was maintained by pre-mixinginto the formulation monodisperse glass microspheres. The cell wasirradiated with UV light at an intensity of 100 mW/cm² 365 nm for 500seconds. While the cell was being irradiated, an electric field of 111V60 Hz was applied to the cell. Optical Properties Formulation ScatteringState - Clear State - Example (% by weight) Haze (%) Clarity (%) 4 92.9%TEB 50 102.0 97.8  5.0% EGDMA  2.0% ZLI-4571  0.1% Irgacure 819

Examples 5A-5I

A bistable mode cell was prepared by using each of the followingcompositions (% by weight listed): Threshold Optical Frequency PropertyExample Formulation (kHz) Haze Clarity Bistability 5A 97.8%ZLI-4788-000¹¹ 2.5 Good Good Fair  2.0% ZLI-4572¹²  0.2%tetrabutylammonium tetraphenyl borate 5B 97.8% ZLI-4788-000 2.5 GoodGood Fair  2.0% ZLI-4572  0.2% 006 g 2-propylisoquinolinium bromide 5C97.8% ZLI-4788-000 20 Good Good Good  2.0% ZLI-4572  0.2%Tetrabutylammonium bromide 5D 97.8% ZLI-4788-000 25 Good Good Good  2.0%ZLI-4572  0.2% Tetrabutylammonium hexafluoro phosphate 5E 97.8%ZLI-4788-000 25 Good Good Good  2.0% ZLI-4572  0.2% Tetrabutylammoniump-toluene sulfonate 5F 97.8% ZLI-4788-000 20 Good Good Good  2.0%ZLI-4572  0.2% Tetrakis(decyl)ammonium Bromide 5G 97.8% ZLI-4788-000 2Good Good Good  2.0% ZLI-4572  0.2% Tetrahexadecylammonium Bromide 5H97.0% ZLI-4788-100¹³ 1 Good Good No  2.0% ZLI-4572  1.0%1-bromohexadecane 5I Same as 5H, but the substrates were treated by 1Good Good No polyimide SE-1211¹¹Negative type liquid crystal, Merck, Germany¹²Chiral compound, Merck, Germany¹³Negative type liquid crystal, Merck, Germany

The composition was filled into a cell having two glass substratescoated with indium-tin oxide on their inner surfaces. (In Example 5I,the conductive ITO layers were treated with polyimide.) The glasssubstrates were separated by 20 micron spacers. A 50-80 volt electricfield (square or sine waveform) was applied for about 1 second to drivethe panels. A frequency of 60 Hz was used to switch the cell to ascattering state. Frequencies higher than the threshold frequency wasrequired to switch the cell to an optically clear state. The lower thefrequency, the less the power requirement to drive the device.

The bistability rating is based on the length of time the haze stateremained after switching. An indication of “good” means that the cellremained hazy for at least a day or longer; an indication of “fair”means that the cell remained hazy for between one-half hour to severalhours; and an indication of “no” means that the cell remained hazy forless than 10 minutes.

Example 6

A bistable mode cell was prepared substantially in accordance withExample 5A-5J above, with the exception that flexible transparentsubstrates were used. The liquid crystal formulation was sandwichedbetween two 7 mil PET films having a conductive indium-tin oxide coatingon their inner surfaces. A uniform cell gap of 25 microns was maintainedby pre-mixing into the formulation monodisperse glass microspheres.Threshold Optical Frequency Property Example Formulation (kHz) HazeClarity Bistability 6 97.7% ZLI-4788-000 2.0 Good Good Fair  2.0%ZLI-4572  0.3% 2-propylisoquinolinium bromide

Examples 7A-7M

A bistable mode cell was prepared by using each of the followingcompositions containing a mesogenic monomer (% by weight listed). Themesogenic monomer (RM82 from Merck) has the following structure:

Threshold Optical Frequency Property Example Formulation (kHz) HazeClarity Bistability 7A 95.7% ZLI-4788-000 3 Good Fair Good  2.0%ZLI-4571  0.1% tetrabutylammonium tetraphenyl borate  2.0% reactivemesogen RM82  0.2% photoinitiator Benzoin methyl ether 7B 95.6%ZLI-4788-000 2.5 Good Fair Good  2.0% ZLI-4571  0.2% tetrabutylammoniumtetraphenyl borate  2.0% RM82  0.2% Benzoin methyl ether 7C 97.2%ZLI-4788-000 2.5 Good Good Fair  2.0% ZLI-4571  0.2% tetrabutylammoniumtetraphenyl borate  0.5% RM82  0.1% Benzoin methyl ether 7D 96.7%ZLI-4788-000 2 Good Good Fair  2.0% ZLI-4571  0.2% tetrabutylammoniumtetraphenyl borate  1.0% RM82  0.1% Benzoin methyl ether 7E 94.5%ZLI-4788-000 4 Good Good No  2.0% ZLI-4571  0.2% tetrabutylammoniumtetraphenyl borate  3.0% RM82  0.3% Benzoin methyl ether 7F 95.5%ZLI-4788-000 25 Good Good Good  2.0% ZLI-4572  0.3%2-propylisoquinolinium bromide  2.0% RM82  0.2% Benzoin methyl ether 7G95.5% ZLI-4788-000 0.8 Fair Good Good  2.0% ZLI-4572  0.3%1-heptyl-4(4-pyridyl)-pyridinium bromide  2.0% RM82  0.2% Benzoin methylether 7H 94.8% ZLI-4788-000 4 Fair Good Good  2.0% ZLI-4572  1.0%2-propylisoquinolinium-tetraphenyl borate  2.0% RM82  0.2% Benzoinmethyl ether 7I 95.5% ZLI-4788-000 5 Good Good Good  2.0% ZLI-4571  0.3%Dodecyl pyridinium tetraphenyl borate  2.0% RM82  0.2% Benzoin methylether 7J 95.2% ZLI-4788-000 0.5 Good Good Good  2.0% ZLI-4571  1.0%1-bromohexadecane  1.5% RM82  0.3% Benzoin methyl ether 7K 87.6%ZLI-4788-000 5 Fair Good Good  9.0% CB15  1.0% Tetrabutylammoniumtetraphenyl borate  2.0% RM82  0.4% Benzoin methyl ether 7L Same as 7K,but filled into panels consisting of 5 Good Good Good polyimide SE-3510coated glass substrates 7M Same as 7K, but filled into panels consistingof 5 Fair Fair Fair polyimide SE-1211 coated glass substrates

The composition was filled into a cell having two glass substratescoated with indium-tin oxide on their inner surfaces. (In Examples 7Land 7M, the conductive ITO layers were treated with polyimide.) Theglass substrates were separated by 20 micron spacers. The polymerizablecompositions were cured by exposing the cells to irradiation with UVlight at an intensity of 5 mW/cm² at 365 nm for 200 seconds. Beforecuring, the cells were turned to an optically clear state by applying ahigh frequency field, e.g., 50 Vrms at, e.g., 3 kHz, or other means thatinclude the planar surface alignment. A 50-80 volts electric field(square or sine waveform) was applied for about 1 second to drive thecells.

Some cells were actually driven by 25 volts electrical field. Typically,60 Hz electric field turns the cells to a hazy state. An electric fieldhaving a frequency higher than the threshold frequency switches thecells to an optically clear state.

Example 8

A bistable mode cell was prepared substantially in accordance withExamples 7A-7M above, with the exception that flexible transparentsubstrates were used. The liquid crystal formulation was sandwichedbetween two 7 mil PET films having a conductive indium-tin oxide coatingon their inner surfaces. A uniform cell gap of 25 microns was maintainedby pre-mixing into the formulation monodisperse glass microspheres. Theliquid crystal composition was cured by exposing the cell to UV lighthaving an intensity of 5 mW/cm² at 365 nm for 200 seconds. Prior tocuring, the cells were turned to an optically clear state by applying ahigh frequency field. A 50-80 volt electric field (square or sinewaveform) was applied for about 1 second to drive the cell. A frequencyof 60 Hz was used to switch the cell to a scattering state. A frequencyhigher than the threshold frequency was required to switch the cell tothe optically clear state. Threshold Optical Frequency Property ExampleFormulation (kHz) Haze Clarity Bistability 8 95.5% ZLI-4788-000 3.0 GoodFair Good  2.0% g ZLI-4572  0.3% 2-propylisoquinolinium bromide  2.0%RM82  0.2% Benzoin methyl ether

While not wishing to be bound by theory, it is believed that theaddition of an ionic compound improves the stability of the bistableelectro-optical device. It is believed that the doping of the ioniccompound reduces the conductivity and hence increases the thresholdfrequency to a desirable range. The nature of ionic conductivity differsfrom electron conductivity and responds differently to the frequency ofthe applied electric field.

At low frequency, e.g., 60 Hz, the conductivity assumes the dominantrole. Due to the positive ionic conductivity anisotropy of the liquidcrystal mixture, the liquid crystal molecules tend to align along theexternal electric field. Such molecular alignment favors the focal conictexture, i.e., an optically scattering state. As one of the stablestates of the cholesteric texture, the hazy state can be kept without anapplied electric field.

At the higher frequency, i.e., above the threshold frequency, thedielectric behavior is believed to be more dominant. Due to the negativedielectric anisotropy of the liquid crystals, the liquid crystalmolecules tend to align perpendicularly to the external electric field,i.e., build in the planar texture. The planar texture is transparent tothe visible light. It is the optically clear state. The clear state isanother stable cholesteric texture that can be maintained without anapplied field for a long time.

While the invention has been explained in relation to its preferredembodiments, it is to be understood that various modifications thereofwill become apparent to those skilled in the art upon reading thespecification. Therefore, it is to be understood that the inventiondisclosed herein is intended to cover such modifications as fall withinthe scope of the appended claims.

1. A bistable electro-optical device comprising: a first transparentsubstrate, having an interior and exterior surface; a second transparentsubstrate, having an interior and exterior surface; a transparentelectrically conductive layer on the interior surface of each of thefirst and second flexible substrates; a cholesteric liquid crystalmaterial comprising a nematic liquid crystal material, a chiral materialand an ionic additive, wherein the cholesteric liquid crystal materialis positioned between the electrically conductive layers of the firstand second transparent flexible substrates; wherein the first and secondtransparent substrates are spaced apart a predetermined distance; andwherein the cholesteric liquid crystal material is switched from atransparent state to a light scattering state and from a lightscattering state to a transparent state upon application of an electricfield to the cholesteric liquid crystal material, and wherein anelectric field is not required to maintain either the transparent stateor the light scattering state.
 2. The electro-optical device of claim 1wherein the cholesteric liquid crystal material comprises a polymermatrix having cholesteric liquid crystals stabilized and supportedtherein.
 3. The electro-optical device of claim 2 wherein the polymermatrix is formed from a polymerizable monomer or polymer.
 4. Theelectro-optical device of claim 3 wherein the polymer matrix is formedfrom the UV curable resin comprising:


5. The electro-optical device of claim 1 wherein the cholesteric liquidcrystal material comprises about 90-99% by weight nematic liquid crystalmaterial, about 0.5-3% by weight of chiral dopant and about 0.05-0.5% byweight of ionic additive.
 6. The electro-optical device of claim 1wherein the ionic additive comprises an ionic compound.
 7. Theelectro-optical device of claim 6 wherein the ionic compound is selectedfrom the group consisting of 1-heptyl-4(4-pyridyl) pyridinium bromide,1-phenacyl pryridinium bromide, 2-propylisoquinolinium bromide,2-propylisoquinolinium tetraphenyl borate, cetylpyridinium bromide,dodecyl pyridinium tetraphenyl borate, tetrabutyl ammonium bromide,tetrabutyl ammonium p-toluene sulfonate, tetrabutylammonium hexafluorophosphate, tetrabutylammonium tetraphenyl borate, tetrahexadecylammoniumbromide, tetrahexadecylammonium hexafluorophosphate,tetrakisdecylammonium bromide, tetrakisdecylammoniumhexafluorophosphate, tetrahexadecylammonium tetraphenyl borate,tetrakisdecylammoniym tetraphenyl borate and mixtures thereof.
 8. Theelectro-optical device of claim 1 wherein the ionic additive comprises apolar compound.
 9. The electro-optical device of claim 1 wherein theionic additive comprises a charge transfer agent.
 10. Theelectro-optical device of claim 1 wherein the first and secondtransparent substrates comprise a flexible polymeric film.
 11. Theelectro-optical device of claim 1 wherein the interior surface of atleast one of the first and second transparent substrates has a pluralityof alignment surface profiles thereon.
 12. The electro-optical device ofclaim 8 wherein the surface profiles comprise a plurality of groove. 13.The electro-optical device of claim 1 wherein the interior of theconductive layer of at least one of the first and second transparentsubstrates has a plurality of alignment surface profiles thereon. 14.The electro-optical device of claim 1 wherein the cholesteric liquidcrystal material is switched from the transparent state to the lightscattering state upon application of a low frequency voltage andswitched from the opaque state to the transparent state upon applicationof a high frequency voltage.
 15. The electro-optical device of claim 1further comprising a protective layer on at least one of the first andsecond flexible substrates.
 16. The electro-optical device of claim 1further comprising a barrier layer on at least one of the first andsecond flexible substrates.
 17. The electro-optical device of claim 16wherein the barrier layer comprises a moisture barrier layer.
 18. Theelectro-optical device of claim 16 wherein the barrier layer comprises aUV blocking layer.
 19. The electro-optical device of claim 1 wherein thecholesteric liquid crystal material has negative dielectric anisotropy.20. The electro-optical device of claim 1 wherein the structure is in arolled configuration.
 21. The electro-optical device of claim 1 whereinthe cholesteric liquid crystal material further comprises a plurality ofspacers.
 22. The electro-optical device of claim 1 wherein thecholesteric liquid crystal material further comprises a dye.
 23. Theelectro-optical device of claim 2 wherein the cholesteric liquid crystalmaterial comprises discrete regions of liquid crystals stabilized withina polymer matrix separated by polymeric walls extending from theinterior surface of the first transparent substrate to the interiorsurface of the second transparent substrate.
 24. The electro-opticaldevice of claim 23 wherein the polymeric walls comprise a polymer matrixcontaining liquid crystals; and wherein the walls are electricallyresponsive to the application of an electric field.
 25. Theelectro-optical device of claim 1 wherein the distance between the firstand second transparent substrates is greater than about 10 microns. 26.The electro-optical device of claim 23 wherein the polymeric wallssupport the discrete regions of liquid crystals stabilized within thepolymer matrix and maintain the predetermined distance between the firstand second transparent substrates.